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A University of Toronto-led team of astronomers has discovered over two dozen new free-floating brown dwarfs -- space objects that straddle the boundary between stars and planets --  that reside in two young star clusters. What's more, one cluster contains a surprising surplus of them, harbouring half as many of these astronomical oddballs as normal stars.

The researchers were interested to find that the dwarfs form like stars but have masses that are similar to planets. Sometimes described as failed stars, they glow brightly when young from the heat of formation, but cool down over time and end up with atmospheres that exhibit planet-like characteristics.

"Our findings suggest once again that objects not much bigger than Jupiter could form the same way as stars do. In other words, nature appears to have more than one trick up its sleeve for producing planetary mass objects," said Professor Ray Jayawardhana, Canada Research Chair in Observational Astrophysics at the University of Toronto and leader of the international team that made the discovery.

"If nature can produce planetary mass objects in multiple ways, that adds to the remarkable diversity of worlds we're finding out there."

They are also curious about the large number of brown dwarfs found in one of the star clusters, but not the other.

"Brown dwarfs seem to be more common in NGC 1333 than in other young star clusters. That difference may be hinting at how different environmental conditions affect their formation," said University of Toronto’s Koraljka Muzic, lead author of the second paper.

The findings come from observations using the Subaru Telescope in Hawaii and the Very Large Telescope (VLT) in Chile during the Substellar Objects in Nearby Young Clusters (SONYC) survey. Astronomers took extremely deep images of the NGC 1333 and rho Ophiuchi star clusters with Subaru at both optical and infrared wavelengths. Once they identified candidate brown dwarfs from the very red colors, the research team confirmed them with spectra taken at Subaru and the VLT. The team's findings will be reported in two upcoming papers in the Astrophysical Journal and presented this week at a scientific conference in Garching, Germany.

A six-Jupiter-mass brown dwarf found in the NGC 1333 cluster is one of the least massive free-floating objects known.

"Its mass is comparable to those of giant planets, yet it doesn't circle a star. How it formed is a mystery," said Aleks Scholz of the Dublin Institute for Advanced Studies in Ireland, lead author of one paper and a former postdoctoral fellow at the University of Toronto.

Several other newly identified brown dwarfs in both NGC 1333 and rho Ophiuchi clusters have masses below 20 times that of Jupiter.

"We could not have made these exciting discoveries if not for the remarkable capabilities of Subaru and the VLT. Instruments that can image large patches of the sky and take hundreds of spectra at once are key to our success," said co-author Motohide Tamura of the National Astronomical Observatory of Japan.

Other co-authors of the two papers are Vincent Geers of ETH Zurich in Switzerland, also a former U of T postdoc, and Mariangela Bonavita of the University of Toronto.